Transient voltage-frequency coupling control method and system for distributed photovoltaic access power distribution network

Abstract

This invention discloses a transient voltage-frequency coupling control method and system for distributed photovoltaic (PV) grid integration into a distribution network, belonging to the field of distribution network stability control technology. Addressing the technical bottleneck of reduced grid inertia and prominent voltage-frequency coupling issues in the distribution network caused by large-scale distributed PV (DPV) integration, this invention, based on the IEEE 33-node distribution network model, triggers phased reactive power compensation for DPV low-voltage ride-through (LVRT) by real-time monitoring of fault conditions: reactive power compensation is initiated when the voltage drops to a threshold, and switches to forced reactive power compensation (reducing active power output to free up reactive power capacity) after reaching the converter current limit; simultaneously, the active power change on the distribution network side is quantified, and the active power output of the main grid is dynamically adjusted in conjunction with the grid inertia parameters to maintain system active power balance. This invention effectively suppresses transient voltage drops and frequency fluctuations, breaks the traditional independent voltage and frequency control mode, is compatible with existing equipment, and is applicable to scenarios with different DPV penetration rates and output ratios, providing key technical support for the stable operation of high-penetration renewable energy distribution networks.

Description

Technical Field

[0001] This invention relates to the field of distribution network stability control technology, specifically to a transient voltage-frequency coupling control method and system for distributed photovoltaic power grid integration. Background Technology

[0002] With the increasing penetration of new energy sources such as distributed photovoltaic (DPV) in power distribution networks, the structure and operating characteristics of traditional power distribution systems are undergoing profound changes. New energy sources are connected to the grid via power electronic devices, replacing traditional synchronous machines, leading to reduced system inertia, decreased strength, and weakened disturbance immunity. When the main grid's inertia is insufficient, AC faults in the distribution network can easily cause severe voltage drops and continuous fluctuations, and voltage and frequency exhibit strong coupling characteristics. Traditional methods of independently controlling voltage or frequency can no longer meet the requirements for stable system operation.

[0003] Existing research largely focuses on the independent impact of photovoltaic grid connection on transient voltage or frequency, or on control strategies for single stability aspects. It lacks in-depth exploration of the transient voltage-frequency coupling mechanism in distribution networks under low inertia conditions. This leads to limitations in voltage regulation and poor frequency fluctuation suppression in high-penetration DPV access scenarios. Therefore, a coupled control scheme that can adapt to the characteristics of low-inertia distribution networks and balance voltage and frequency stability is urgently needed. Summary of the Invention

[0004] Purpose of the invention: The present invention aims to solve the transient voltage-frequency coupling problem that occurs after high-penetration DPV is connected to a low-inertia distribution network, and to provide a control method and system that can coordinately control voltage drop and frequency fluctuation and maintain stable system operation.

[0005] Technical solution: A transient voltage-frequency coupling control method for distributed photovoltaic power grid integration, comprising the following steps: (1) Monitor the operation status of the distribution network containing distributed photovoltaics in real time, and start the transient response monitoring process when a preset type of fault is detected in the distribution network; (2) Based on the low-voltage ride-through characteristics of distributed photovoltaics, when the distribution network voltage drops to the preset voltage threshold, the DPV controller switches to the LVRT reactive power compensation strategy and injects reactive current according to the following calculation formula. The compensation lasts for a preset duration:

[0006] In the formula, For the preset compensation coefficient, The rated voltage of the power grid. This is the actual voltage of the power grid. This refers to the rated current of the DPV converter. (3) When the reactive power compensation reaches the maximum allowable current of the converter If the distribution network voltage fails to recover to the preset recovery threshold, the reactive power forced compensation strategy will be switched to reduce the active power output current of the DPV. Free up capacity to increase reactive power output current. ,satisfy ; (4) Monitor and quantify the changes in active power on the distribution network side in real time. Combined with the generalized inertia constant of the main network and total system capacity Dynamically adjusting the main grid's active power output maintains system active power balance and suppresses frequency fluctuations. This adjustment is based on the quantization results of the entire frequency control system, which counteracts active power imbalances. The quantization results of the entire frequency control system are determined by the current frequency offset amplitude. To calculate:

[0007] In the formula, It is the total active power imbalance of the system. This is the system's rated frequency.

[0008] Furthermore, the distribution network described in step (1) conforms to the IEEE 33 node requirements, and the transient response characteristics of the DPV are determined by its transient control strategy, configured as follows: When the voltage drops to the voltage threshold for entering the LVRT, the photovoltaic controller switches to the LVRT reactive power compensation strategy, and the strategy switch takes 10~20 ms to complete. When the reactive power compensation is completed or the compensation has reached the maximum capacity of the converter, if the voltage has not recovered to the specified value, the reactive power forced compensation strategy will be switched to continue to increase reactive power output. If the converter capacity is insufficient, the active power output is reduced to free up capacity for reactive power generation.

[0009] Furthermore, in step (2), the DPV enters the LVRT to increase the reactive current. The value helps the system maintain voltage stability; the compensation degree is set to the actual grid voltage. Each drop in grid rated voltage 0.05-0.1 times, preset compensation coefficient Values ​​range from 1.2 to 2.

[0010] Furthermore, the preset recovery threshold mentioned in step (2) is 90% to 95% of the rated voltage.

[0011] Furthermore, the maximum allowable current of the converter mentioned in step (3) Rated current of DPV converter 1.1 times.

[0012] Furthermore, the change in active power on the distribution network side mentioned in step (4) ,in Contributing to DPV For active power loss in distribution network lines, This refers to the active power consumption of the distribution network load.

[0013] Step (4) involves dynamically adjusting the active power output of the main network, which includes the following specific processes: First, calculate the current frequency offset. ,like If the value is negative, the main grid needs to increase its active power output to offset the total active power imbalance of the system. ;like If the result is positive, the main grid needs to reduce its active power output to offset the total active power imbalance in the system. According to the system control requirements, the adjusted total active power imbalance of the system is achieved. It tends towards 0.

[0014] Furthermore, the method also includes dynamically optimizing control parameters based on DPV penetration rate, defining DPV penetration rate as the ratio of total installed DPV capacity to the maximum load power of the distribution network, denoted as... ; when At that time, the compensation coefficient in step (2) will be... The value is adjusted to 1.8~2.0, and the preset voltage threshold is lowered to 0.80~0.85pu, thus activating the LVRT reactive power compensation strategy in advance. when At that time, the compensation coefficient will be The value is adjusted to 1.2~1.5, and the preset voltage threshold is kept at 0.85pu to ensure the stability of the active power output of DPV.

[0015] Based on the implementation of the above method, the present invention also provides a transient voltage-frequency coupling control system for distributed photovoltaic power grid integration, the system comprising: Distribution network configuration module: used to adapt to distribution networks with different topologies, configure the installation location, installed capacity and operating parameters of DPV, and set the distribution network reference parameters and line electrical parameters; Fault monitoring module: used to collect voltage, current and frequency signals of the distribution network in real time, identify preset types of faults and trigger transient response monitoring; DPV Cooperative Control Module: Includes a reactive power graded compensation unit and a power coordination unit. The reactive power graded compensation unit is used to switch between reactive power fixed compensation and reactive power forced compensation strategies according to the voltage drop level. The power coordination unit is used to adjust the ratio of active and reactive power output current of DPV. System balance adjustment module: used to quantify changes in active power on the distribution network side. The frequency offset is calculated by combining the inertial characteristic parameters of the main grid, and the active power output of the main grid is dynamically adjusted to maintain the active power balance of the system. Feedback Adaptation Module: Used to collect electrical signals in the transient process in real time, providing feedback basis for control strategy switching and parameter adjustment, and adapting to different DPV penetration rates and operating scenarios.

[0016] Beneficial effects: Compared with existing technologies, this invention clarifies the dominant role of DPV transient response in the voltage-frequency coupling process, effectively suppresses voltage drops through a phased reactive power compensation strategy, and improves the transient voltage stability of the distribution network. Secondly, based on the active power balance principle, it dynamically adjusts the main grid output, solving the frequency fluctuation problem caused by DPV voltage regulation behavior and breaking the limitations of traditional independent voltage and frequency control. Thirdly, the control strategy is compatible with the inherent low-voltage ride-through characteristics of DPV, requiring no major modifications to existing equipment, and has strong engineering practicality. Finally, this invention is applicable to scenarios with different DPV penetration rates and output ratios, effectively improving the stable operation level of high-penetration renewable energy distribution networks and providing technical support for large-scale renewable energy integration. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the voltage-active power-frequency coupling characteristics in the method of the present invention; Figure 2 This is a control logic diagram constructed using an IEEE 33-node distribution network as an example. Figure 3 This is the bus voltage waveform under the influence of inertia and DPV; Figure 4 The system frequency waveform is under the influence of inertia and DPV; Figure 5 It is the transient response waveform of the DPV parameters; Figure 6 This explains the principle by which DPV current injection affects voltage. Figure 7 These are transient voltage waveforms under different permeability levels; Figure 8 These are transient frequency waveforms at different permeabilities; Figure 9 These are voltage waveforms with different output percentages at a penetration rate of 110%. Figure 10 It is a frequency waveform with different output ratios at a penetration rate of 110%. Detailed Implementation

[0018] The penetration rate of new energy sources, represented by distributed photovoltaic (DPV), in distribution networks is continuously increasing. Large-scale DPV grid connection can adversely affect the transient voltage stability of the system. New energy power generation connects to the grid through power electronic devices, replacing traditional synchronous machines (inertial energy sources), leading to a reduction in inertia, a significant decrease in system strength, and a weakened ability to withstand disturbances. Especially when the main grid's inertia support is insufficient, AC faults in the distribution network can easily cause severe voltage drops and continuous fluctuations, and strong dynamic interactions occur between the system and the main grid. Traditional analysis methods are insufficient to accurately describe these transient processes. The coupling characteristics of voltage and frequency have become a key issue for the safe and stable operation of distribution networks under the background of large-scale DPV integration.

[0019] Based on this problem, this invention addresses scenarios with insufficient grid inertia and aims to conduct in-depth research on the transient voltage-frequency coupling characteristics after DPV is connected to the distribution network. Therefore, it establishes a transient voltage-frequency coupling control method and system for distributed photovoltaic (PV) grid connection. Figure 1 As shown, the above technical solution can effectively suppress transient voltage drops and frequency fluctuations, break the traditional independent voltage and frequency control mode, be compatible with existing equipment, and be applicable to different DPV penetration rates and power output ratios, providing key technical support for the stable operation of high-penetration new energy distribution networks.

[0020] This invention monitors voltage changes under fault conditions and utilizes the low-voltage ride-through characteristic of a distributed photovoltaic (DPV) to inject reactive power in stages. Simultaneously, it quantifies changes in active power on the distribution network side and dynamically adjusts the active power output of the main grid, achieving coordinated and stable control of voltage and frequency. Specifically, when a fault in the distribution network causes a voltage drop, the DPV first rapidly injects reactive current to support the voltage using a reactive power constant compensation strategy. Once the converter current limit is reached, it switches to a reactive power forced compensation strategy, further enhancing reactive power support capability by adjusting active power output. Simultaneously, based on the active power balance principle, it suppresses frequency fluctuations through dynamic adjustment of the main grid's active power output, ultimately achieving coupled control of transient voltage and frequency.

[0021] This embodiment uses DIgSILENT / PowerFactory to build an IEEE 33-node distribution network, the structure of which is as follows. Figure 2 As shown, Figure 2 The distribution network shown has 33 voltage nodes (branches). The initial reference voltage is 10 kV, the power reference is 12 MW active power and 5.5 MVar reactive power, and the power factor is 0.9. The distribution lines are all connected by cables, i.e., the network R≈X, and the connected main grid is a system with a high proportion of renewable energy output.

[0022] The above technical solution is verified by simulation test. Taking the IEEE 33-node distribution network as an example, a phase-to-phase fault occurs between nodes 24 and 25 when the fault setting is 0.2 s, with a transition resistance of 2 Ω.

[0023] DPV installation locations are nodes 5, 9, 14, 17, 21, 25, 28, and 31. Currently, it is generally not allowed for the distribution network to transmit distributed energy power. The installed capacity of DPV is set to 1.1 times the maximum load power, that is, the penetration rate is 110%, and the steady-state active power output limit is set to 90% of the installed capacity.

[0024] A phase-to-phase fault is set to occur between nodes 24 and 25 at 0.2 s, with a transition resistance of 2 Ω. Considering the fault clearing time limit, the maximum observation time limit is set to 1 s.

[0025] Under the influence of inertia and DPV, the voltage and system frequency changes at node 1 (low-voltage bus) when a fault occurs in the distribution network are as follows: Figure 3 and Figure 4 As shown. Figure 3 It can be seen that before and after the decrease in inertia, the voltage drops to its lowest point at 0.2066 s, with the voltage drop being even lower for systems with low inertia. After the voltage drop, the bus voltage of systems without DPV access enters transient stability, while the bus voltage of systems with DPV access experiences a brief and sharp rebound at 0.213 s, ending its rise at 0.2212 s, and then slowly drops. Figure 4 It is evident that low-inertia systems without DPV exhibit slow frequency shifts under fault conditions, resulting in poorer frequency stability. With DPV integrated into both systems, the frequency shift of low-inertia systems becomes more severe, and the relative recovery rate is slower.

[0026] The fault occurred at 0.2 seconds, and Figure 3 The frequency change begins at 0.213 s, indicating that this change is not directly caused by a fault. When the voltage drops to a certain level, the DPV will enter Low Voltage Ride-Through (LVRT) switching mode for voltage regulation, but it lacks a frequency response mechanism. Therefore, the frequency change is a disturbance introduced by the DPV to adjust the voltage. Taking the 21-node DPV as the observation object, the changes in its parameters before and after entering LVRT voltage regulation are as follows... Figure 5 As shown. Figure 5 It can be seen that the DPV maintains a constant output current in steady state and transient instant, and its output power changes linearly with voltage drop. When the LVRT switching strategy is used for voltage regulation, two complex power change processes occur.

[0027] and Figure 4 , Figure 5The voltage and frequency changes shown are related. After the DPV is connected to the distribution network, starting at 0.213 s, the reactive and active power of the DPV suddenly increases, accompanied by a voltage rise and a frequency decrease; at 0.2212 s, the reactive output continues to increase while the active output decreases. During this stage, the voltage slowly decreases while the frequency, although gradually recovering, remains in a low-frequency state.

[0028] In summary, the transient response characteristics of DPV greatly influence the development of transients, not only dominating the voltage changes during transient processes but also further perturbing the frequency, resulting in voltage-frequency coupling characteristics in the system.

[0029] Furthermore, to clarify the voltage-frequency coupling characteristics of the system, the fault response characteristics of each electrical component should be analyzed first, as well as the interaction behavior between the system and components with active capabilities, to clarify the voltage-frequency disturbance path.

[0030] The transient response characteristics of a DPV (Digital Photovoltaic) system are determined by its transient control strategy. Currently, a widely used DPV transient control strategy is as follows: when the voltage drops to the threshold voltage for entering the LVRT (typically 0.85 pu), the photovoltaic controller switches to the LVRT reactive power compensation strategy, which takes 10-20 ms to complete. If the voltage does not recover to the specified value after reactive power compensation is completed or the converter's maximum capacity has been reached, the system switches to a forced reactive power compensation strategy to continue generating reactive power. If the converter capacity is insufficient, the active power output is reduced to free up capacity for reactive power generation.

[0031] In response, combining DPV's transient strategies with... Figure 5 The transient response characteristics of the DPV are obtained from the waveforms of the electrical quantity changes shown below: After the fault occurs at 0.2 s, During a sharp decline, PDG follows suit. Because the primary control objective during the transient period is to increase... Compensation was ineffective, and the transient strategy failed to address the issue. Before control is applied, its value changes slowly under the control of the inner current loop; during this brief period, it can be considered a constant. During periods of slow change, the voltage characteristics of the DPV are similar to those of the load.

[0032] Due to the inherent characteristics of PLL, it enters a transient state. A drastic drop in voltage may also cause a PLL tracking delay, preventing it from tracking the grid voltage again. , A value that is not zero will cause power fluctuations, because it is relatively... , When the reactive power is relatively small, the active power disturbance is not obvious, but since the reactive power itself is 0, a significant disturbance will occur, such as... Figure 5As shown, the corresponding reactive power curve fluctuates between 0.2 s and 0.2066 s.

[0033] like Figure 5 As shown, DPV enters LVRT increase at time 0.213 s. The value helps the system maintain voltage stability; the compensation degree is generally equal to the grid voltage Ugrid for every drop below the grid rated voltage. 0.1 times the rated current of the converter is provided. The reactive power is compensated by 0.2 times the current value, as shown in the following formula:

[0034] In the formula, =2, with a value range of 1.2~2.

[0035] The reactive power compensation limit is mainly limited by the maximum allowable current of the converter. During transient conditions, the rated current of the converter should be considered. 1.1 times, which must satisfy:

[0036] The fault voltage drop studied further in this invention is all below 0.45 pu, and the reactive power compensation will directly reach the maximum allowable current of the converter.

[0037] The principle of reactive power regulation by DPV is the same as the principle of how injected power at nodes affects the voltage of each node, such as... Figure 6 In Add one DPV injects additional reactive power at the grid connection point, which will improve the voltage level of the line and even the entire distribution network.

[0038] If the voltage does not reach the specified value after the reactive power compensation period ends, the system will switch back to the forced reactive power compensation strategy and continue to increase the voltage. However, if the current has reached the converter's maximum allowable current, then a forced reduction is required. Used to improve Overall, the above equation is satisfied.

[0039] During this stage, voltage changes are complex because the distribution network cannot be PQ decoupled. Taking node 21 as an example, substituting the line impedance... Load power factor =0.9, therefore:

[0040] In the formula, due to and If all values ​​are constants, then this expression is a single-variable function. hour, A maximum value appears: .

[0041] Further analysis, The time function is monotonically increasing, when If the time function decreases, then after the output current reaches the maximum allowable current of the converter, since Start reducing until the converter's full capacity is reached. The time period between, based on The interval where it is located It will first grow larger and then smaller, or it may even start to shrink immediately.

[0042] During the transient process, the DPV does not actively adjust the frequency or respond to frequency disturbances, but the voltage regulation behavior of the DPV and its interaction with various electrical components of the distribution network cause disturbances in active power, which in turn leads to frequency fluctuations.

[0043] Interconnected power grids should operate synchronously; therefore, frequency issues are a systemic problem related to the power system as a whole. The system frequency response formula is:

[0044] In the formula The system's rated frequency; The generalized inertial constant is the ratio of the system's total inertial capacity to its total capacity. Total system capacity; This refers to the system's active power imbalance. This represents the total inertial capacity of the system.

[0045] Further determine A non-zero value will cause frequency changes and affect the system's total inertial capacity. Under certain circumstances, | The larger the value, the faster the rate of frequency change; the system's active power imbalance. Composed of all active power supply and demand in the system, it can be summarized into the following parts:

[0046] Based on the above analysis, It is the system's inertial energy that generates active power. It is the system's new energy source (without inertia) that generates active power. It is the active power consumed by the load. It is the active power lost when current flows through equipment within the network. The main network is summarized as follows: It serves as an equivalent power source with a certain inertia to supply power to the distribution network, while the rest is the distribution network side (load side).

[0047] Based on the principle of the transient effect of DPV on voltage obtained from the aforementioned analysis, it can be seen that voltage changes will inevitably lead to voltage fluctuations on the distribution network side of the equation. , Changes in bus voltage The fault point is affected by the voltage regulation change of DPV. While remaining essentially unchanged, the power loss on the line during transmission from node 1 to the fault point will inevitably change, leading to... change.

[0048] The overall active power changes in the distribution network will result in system active power imbalance. The system frequency, which is not equal to 0, begins to change. To maintain rated frequency operation, the main network will continuously adjust accordingly. .

[0049] Based on the above, the disturbance path of the system frequency can be summarized as follows: the DPV voltage regulation behavior caused by the fault voltage drop disrupts the system's active power balance, and its continuous power disturbance makes... A value that is consistently non-zero causes frequency disturbances, corresponding to... Figure 3 The low-frequency phase after 0.213 s.

[0050] Once the path of frequency disturbance by voltage is clear, the active power balance state under the common change of various parts in the active power supply and demand composition, based on the above analysis, can clarify the coupling mechanism between voltage and frequency.

[0051] Accordingly, the present invention proposes: 1. Reactive power constant compensation stage: When the distribution network voltage is detected to drop below 0.85 pu, the DPV controller switches to the LVRT reactive power constant compensation strategy and sets... ,according to Injecting reactive current provides compensation for 15ms, quickly supporting voltage recovery. 2. Reactive power forced compensation stage: When the reactive power output current reaches the converter's maximum allowable current. When this happens, the system switches to a reactive power forced compensation strategy, reducing the active power output current. Free up capacity and maintain Continuously improve reactive power support capabilities.

[0052] 3. Active power balance and frequency regulation: Real-time acquisition of active power output of the distribution network side DPV. Line loss and the active power consumption of the distribution network load Calculate the change in active power on the distribution network side. Combining the generalized inertia constant of the mainnet and total system capacity ,pass Calculate frequency changes and dynamically adjust the active power output of the main grid to ensure... Approaching 0 suppresses frequency fluctuations.

[0053] 4. Monitoring Feedback and Strategy Optimization: The monitoring feedback module collects voltage, frequency, active power, and reactive power signals at a sampling frequency of 1kHz to determine the voltage recovery status and frequency fluctuation amplitude in real time. When the voltage recovers to more than 95% of the rated voltage and the frequency fluctuation is less than 0.2% of the rated frequency, the transient control strategy is gradually exited, and the DPV normal operation mode is restored.

[0054] The following analysis uses simulation examples to examine various influencing characteristics.

[0055] 1) The impact of DPV penetration rate on transient voltage and frequency

[0056] With the output ratio remaining constant, the penetration rate was adjusted from 110% to 70%, 30%, and 150% respectively. The voltage and frequency waveforms after the fault are as follows: Figure 7 and Figure 8 As shown, with increased DPV penetration, the transient voltage recovery also increases, but due to voltage-frequency coupling characteristics, this results in a larger frequency shift. The subsequent voltage trend also demonstrates the drawbacks of the DPV reactive power forced compensation stage. When existing reactive power forced compensation measures are applied to this scenario, not only is the voltage regulation capability limited, but also... Negative regulation of voltage occurs at times.

[0057] The changing trend during the frequency recovery phase demonstrates that as DPV penetration increases, frequency stability decreases, changing from a gradual recovery of the rated frequency at 30% penetration to a tendency to approach and then deviate at 150% penetration.

[0058] (2) The effect of DPV output ratio on transient voltage, frequency and voltage regulation sensitivity

[0059] When the DPV penetration rate is set to 110%, and the output ratio is adjusted from 90% to 60% and then to 30%, the voltage and frequency curves after the fault are as follows: Figure 9 and Figure 10 As shown: With a fixed DPV capacity, the less active power output, the greater the available reactive power output capacity during the reactive power compensation period. Figure 9 The voltage rebound was relatively large, but it also caused the reactive power forced compensation stage to directly enter the negative voltage regulation range, changing from the initial rise and fall at 90% output to a direct drop at 30% output.

[0060] As the active power output decreases, the relative amount affected by reactive power voltage regulation decreases, which in turn reduces the frequency offset caused by voltage-frequency coupling during this period.

[0061] At the end of the reactive power compensation phase, the DPV output of 30% is boosted by voltage recovery and autonomous current loop enhancement. Due to the combined effects of these factors, a special situation arises where the active power output exceeds the limit of the DC portion of the electrical energy. To maintain active power balance, The voltage and frequency dropped rapidly, causing both to plummet and severely disrupting their stability.

[0062] In summary, both the penetration rate and output ratio of DPV affect the degree of voltage-frequency coupling in the system; the higher both are, the stronger the coupling characteristics. The presence of DPV makes the transient voltage more stable, but at the same time, the greater the active power output, the greater the change in active power when disturbed under transient conditions, and the greater the impact on the frequency. Combined with the analysis of the system imbalance change, this verifies that the essence of voltage disturbance frequency is the change in active power balance caused by voltage change.

Claims

1. A method for distributed photovoltaic (PV) access to a power distribution network for transient voltage-frequency coupling control, the method comprising: receiving a voltage-frequency coupling control signal from a voltage-frequency coupling control device; and adjusting a power output of a PV power source based on the voltage-frequency coupling control signal. Includes the following steps: (1) Monitor the operation status of the distribution network containing distributed photovoltaics in real time, and start the transient response monitoring process when a preset type of fault is detected in the distribution network; (2) Based on the low-voltage ride-through characteristics of distributed photovoltaics, when the distribution network voltage drops to the preset voltage threshold, the DPV controller switches to the LVRT reactive power compensation strategy and injects reactive current according to the following calculation formula. The compensation lasts for a preset duration: In the formula, For the preset compensation coefficient, This is the rated voltage of the power grid. This is the actual voltage of the power grid. This refers to the rated current of the DPV converter. (3) When the reactive power compensation reaches the maximum allowable current of the converter If the distribution network voltage fails to recover to the preset recovery threshold, the reactive power forced compensation strategy will be switched to reduce the active power output current of the DPV. Free up capacity to increase reactive power output current. ,satisfy ; (4) Monitor and quantify the changes in active power on the distribution network side in real time. Combined with the generalized inertia constant of the main network and total system capacity Dynamically adjusting the main grid's active power output maintains system active power balance and suppresses frequency fluctuations. This adjustment is based on the quantization results of the entire frequency control system, which counteracts active power imbalances. The quantization results of the entire frequency control system are determined by the current frequency offset amplitude. To calculate: In the formula, It is the total active power imbalance of the system. This is the system's rated frequency.

2. The transient voltage-frequency coupling control method for distributed photovoltaic power grid access according to claim 1, characterized in that, The distribution network described in step (1) conforms to the IEEE 33 node requirements. The transient response characteristics of the DPV are determined by its transient control strategy, which is configured as follows: When the voltage drops to the voltage threshold for entering the LVRT, the photovoltaic controller switches to the LVRT reactive power compensation strategy, and the strategy switch takes 10~20 ms to complete. When the reactive power compensation is completed or the compensation has reached the maximum capacity of the converter, if the voltage has not recovered to the specified value, the reactive power forced compensation strategy will be switched to continue to increase reactive power output. If the converter capacity is insufficient, the active power output is reduced to free up capacity for reactive power generation.

3. The transient voltage-frequency coupling control method for distributed photovoltaic power grid integration according to claim 1, characterized in that, In step (2), the DPV enters the LVRT to increase the reactive current. The value helps the system maintain voltage stability; the compensation degree is set to the actual grid voltage. Each drop in grid rated voltage 0.05-0.1 times, preset compensation coefficient The value ranges from 1.2 to 2.

4. The transient voltage-frequency coupling control method for distributed photovoltaic power grid integration according to claim 1, characterized in that, The preset recovery threshold mentioned in step (2) is 90%~95% of the rated voltage.

5. The transient voltage-frequency coupling control method for distributed photovoltaic power grid integration according to claim 1, characterized in that, The maximum allowable current of the converter mentioned in step (3) Rated current of DPV converter 1.1 times.

6. The transient voltage-frequency coupling control method for distributed photovoltaic power grid access according to claim 1, characterized in that, The change in active power on the distribution network side mentioned in step (4) ,in Contributing to DPV For active power loss in distribution network lines, This refers to the active power consumption of the distribution network load.

7. The transient voltage-frequency coupling control method for distributed photovoltaic power grid access according to claim 1, characterized in that, Step (4) involves dynamically adjusting the active power output of the main network, which includes the following specific processes: First, calculate the current frequency offset. ,like If the value is negative, the main grid needs to increase its active power output to offset the total active power imbalance of the system. ;like If the result is positive, the main grid needs to reduce its active power output to offset the total active power imbalance in the system. According to the system control requirements, the adjusted total active power imbalance of the system is achieved. It tends towards 0.

8. The transient voltage-frequency coupling control method for distributed photovoltaic power grid integration according to claim 1, characterized in that, This method also includes dynamically optimizing control parameters based on DPV penetration rate, defining DPV penetration rate as the ratio of total installed DPV capacity to the maximum load power of the distribution network, denoted as... ; when At that time, the compensation coefficient in step (2) will be... The value is adjusted to 1.8~2.0, and the preset voltage threshold is lowered to 0.80~0.85pu, thus activating the LVRT reactive power compensation strategy in advance. when At that time, the compensation coefficient will be The value is adjusted to 1.2~1.5, and the preset voltage threshold is kept at 0.85pu to ensure the stability of the active power output of DPV.

9. A transient voltage-frequency coupling control system for distributed photovoltaic power grid integration, characterized in that, The system includes: Distribution network configuration module: used to adapt to distribution networks with different topologies, configure the installation location, installed capacity and operating parameters of DPV, and set the distribution network reference parameters and line electrical parameters; Fault monitoring module: used to collect voltage, current and frequency signals of the distribution network in real time, identify preset types of faults and trigger transient response monitoring; DPV Cooperative Control Module: Includes a reactive power graded compensation unit and a power coordination unit. The reactive power graded compensation unit is used to switch between reactive power fixed compensation and reactive power forced compensation strategies according to the voltage drop level. The power coordination unit is used to adjust the ratio of active and reactive power output current of DPV. System balance adjustment module: used to quantify changes in active power on the distribution network side. The frequency offset is calculated by combining the inertial characteristic parameters of the main grid, and the active power output of the main grid is dynamically adjusted to maintain the active power balance of the system. Feedback Adaptation Module: Used to collect electrical signals in the transient process in real time, providing feedback basis for control strategy switching and parameter adjustment, and adapting to different DPV penetration rates and operating scenarios.

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